Tiny frogs that have no middle ear use their mouths to hear, French scientists say. A new study in the Proceedings of the National Academy of Sciences shows that the almighty eardrum may not be vertebrates’ only solution for picking up sound.

Gardiner’s Seychelles frogs, known formally as Sechellophryne gardineri, are some of the tiniest amphibians to crawl the Earth, growing to a maximum 11 millimeters long. They and a few other species have evolved in isolation over the last 47 million to 65 million years, and aside from their extreme smallness, the frogs are known for being 'earless' -- they lack a middle ear or an eardrum.

The middle ear is such an important structure that it has evolved independently at least four times in tetrapods, the four-legged animals that gave rise to Earth’s land-dwelling vertebrate lineages, including humans and most other frogs. And yet somehow, certain frog species seemed to be doing just fine without a middle ear, even though 99.9% of sound is reflected by an animal’s skin.

This was confusing to biologists because it meant the frogs should be deaf. And yet, the earless frogs still croaked. Evolutionarily speaking, critters generally don’t communicate with sound unless they can pick it up too. So without a middle ear, how exactly was sound getting through to the frog’s inner ear and then to the brain?

First, the researchers had to make sure the animals really could hear. They recorded frogs' ribbits on Silhouette Island in the Seychelles Archipelago and found they had an average dominant frequency of 5,710 hertz. When they played the calls back, the males responded. Those males didn’t respond to calls from other frog species -- which meant that they could hear well enough to tell the calls apart.

The next step was to figure out how the tiny frog could possibly hear without a middle ear. Perhaps their lungs were capturing the sound, or perhaps it was being transmitted through bone. Using X-ray imaging techniques, the researchers examined how the frogs were built. They found that the lungs were too small to catch the right wavelength of sound, and the bone method didn’t work either. They did find, however, that their oral cavity was big enough that it resonated at 5,738 hertz, capturing sounds at almost exactly the frogs’ average call of 5,710 Hertz.

The frogs were listening with their mouths, the researchers said.

They also compared the earless frog species to its eared peers and found that the tissue between the mouth cavity and the inner ear is much thinner in the earless frog -- roughly 80 μm. There were also fewer layers of tissue, the researchers found, potentially allowing for sound to pass much more easily.

"The presence of a middle ear is not a necessary condition for terrestrial hearing, despite being the most versatile solution for life on land," the authors wrote.